circadian systems

What is a clock made of? We think of springs, gears and moving parts made out of metal. But a clock could, in theory, be designed with almost any material. There are water clocks, sundials, and electromagnetic oscillators that all function to tell time. What difference does it make if the parts are made of liquids, laser beams, or plastic? What if a clock was made of biological material—would it be any less a device for keeping time? Would it surprise you that such clocks exist in your body and in every living thing?